Random call audio drops on 3CX

[myst02]

Hi,

we are experiencing random call audio drops on our 3CX instance that is running behind an OPNsense firewall (virtualized in Proxmox).

When I set up the 3CX instance a few months ago, I followed the official guide for pfSense and haven't changed anything since: https://www.3cx.com/docs/pfsense-firewall/

Everything was working fine until about 2 weeks ago, when this issue suddenly started.
The audio randomly drops for all current calls simultaneously and doesn't come back until the call is terminated. This happens around 3-4 times a day.

I tried updating the OPNsense firewall from 21.1 to the newest 21.7 version, however this didn't help as well. The 3CX Firewall Checker completes without any issues.

When I put the 3CX instance directly behind a public IP without the OPNsense firewall everything works, so I know that this has to be a firewall issue.
I tried doing a packet capture but didn't see anything unusual.

Can anybody help with this issue?

Thanks in advance!